Recently, power generation techniques utilizing a wide variety of natural energy have been developed. As an example, powerful ocean currents, such as the Japan Current, are also energy resources and ocean current electric power generators that generate electric power utilizing the ocean current energy have also been developed. Such an ocean current electric power generator generates electric power by rotating a rotatable wing with the ocean current energy, thereby rotating an electric power generator coupled to the shaft of the rotatable wing.
One type of such ocean current electric power generators is an undersea floating ocean current electric power generator (water floating type ocean current electric power generator), and is disclosed in Patent Document 1, for example. This undersea floating ocean current electric power generator includes an ocean current electric power generator configured as a float with certain buoyancy, and ocean current electric power generator is connected to a mooring cable extending from ocean bottom, such that the ocean current electric power generator generates electric power while remaining under the ocean in the range restricted by the mooring cable. The undersea floating ocean current electric power generator has a simplified structure, without requiring a massive structure, such as a pillar of a wind electric power generator constructed on the ground.
FIG. 12 is a perspective view illustrating an undersea floating ocean current electric power generator disclosed in Patent Document 1. As shown in FIG. 12, this undersea floating ocean current electric power generator is configured as a float 1, and the float 1 is a catamaran type with one float 1 configured from two ocean current electric power generator main bodies 2, 2 and a structure 3 coupling them, for stabilizing the attitude of the float 1. Each ocean current electric power generator main body 2 includes an electric power generator (not shown) inside a nacelle (also referred to as a “pod”) 4, and a rotor shaft (rotation axis) of a rotatable wing 5 is connected to the rotor of the electric power generator.
The nacelles 4 are coupled to left and right ends of the structure 3. The distal ends of mooring cables 6 are coupled to the left-right direction center of the structure 3, and the proximal ends of the mooring cables 6 is moored to an ocean bottom. The float 1 remains under the ocean in the horizontal direction and in the vertical direction in the range restricted by the mooring cables 6. The buoyancy of the float 1 balances the float 1 left and right, and the structure 3 of the float 1 has a wing shape facing the ocean current direction. As a result, the float 1 generates electric power, while changing its orientation in response to a change in the direction of the ocean current, such that the front face (the front face of the rotatable wing 5) thereof opposes to the ocean current direction.
Also, a downstream scheme is employed wherein the rotatable wing 5 is disposed at the rear of the nacelle 4 (the downstream to the ocean current). By disposing the rotatable wing 5 downstream to the nacelle 4 in this manner, it become easier to orient the front face of the float 1 (the front face of the rotatable wing 5) to face the ocean current direction. It is difficult to provide the undersea floating ocean current electric power generator with a driving apparatus that actively controls the yaw direction of the float 1 (controls to align the rotor shaft of the rotatable wing 5 to the ocean current direction). The downstream scheme is employed for that reason, and the nacelles 4 and the structure 3 of the float 1 are shaped to orient to the ocean current direction such that the front face of the float 1 passively faces the ocean current direction.
Further, while the float (undersea floating ocean current electric power generator) 1 moored to the mooring cables 6 remains under the ocean, the float 1 stays in the position where the ocean current force exerted on the float 1, the buoyancy exerted on the float 1, and the tension of the mooring cables 6, are balanced. Specifically, on the float 1, the buoyancy acts vertically upward, the ocean current force acts in the ocean current direction (horizontal direction), and the tension of the mooring cables 6 acts against the buoyancy and the ocean current force. Therefore, if the ocean current force is smaller (i.e., if the current speed of the ocean current is lower), the float 1 ascends to a relatively shallow depth in the ocean. If the ocean current force is greater (i.e., if the current speed of the ocean current is higher), the float 1 descends to a relatively deep depth.
In the meantime, the depth direction profile of the current speed of the ocean current is such that the current speed reduces as the distance from the ocean bottom is smaller in the vicinity of the ocean bottom, and the current speed increases as the distance from the ocean bottom increases. Therefore, when the current speed of the ocean current increases, the float 1 descends down the ocean, and is balanced at a depth where the current speed is appropriately low in the current speed profile in the depth direction. Or, when the current speed of the ocean current becomes smaller, the float 1 ascends in the ocean, and is balanced at a depth where the current speed is appropriately high in the current speed profile in the depth direction.
Patent Document 1 employs the characteristic of the ocean current electric power generator in that the ocean current electric power generator ascends or descends depending on the current speed. It discloses that utilization of such a control for passively and autonomously adjusting the level in the depth direction (depth of water) of the ocean current electric power generator (also referred to as a passive depth control; PDC) eliminates the need for any active control (active depth control).
In the meantime, the rotatable wing 5 includes (generally two or three) blades 5a. A majority of the ocean current electric power generators are proposed as those having a variable pitch angle wing, wherein the wing pitch angles of the blades 5a are adjustable (e.g., Non-Patent Document 1). One of the reasons why a variable pitch angle wing is employed is that a smooth actuation can be achieved with smaller wing projection areas by increasing the wing pitch angle to reduce the input torque to the rotor shaft during an actuation, and subsequently increasing the wing projection areas gradually to gradually increase the number of revolution (revolution speed) of the rotor shaft and the driving power (refer to FIG. 13A).